Quote:
Originally Posted by redpoint5
Since you have a self-exciting alternator, I wouldn't bother expanding your battery capacity. If you get nervous about getting low on charge, or you go on an extended trip, just engage the alternator.
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Yeah, you're probably right Redpoint5. If I wanted to go for cross-country mpg records then a second rear battery would be cool, but for daily use - and it's daily use patterns that determine overall fuel use - then I had already calculated that a single rear battery of 100Ah would be about optimum. The trouble is I kind of DO want to go for cross-country mpg records on occasion (he-he!) because it's only on longer journies that the highest possible mpg is achieveable, ...but I'll resist the temptation. (It's bragging rights really - you know, being able to rock up at a friend's place 200 miles away, ask to plug into his house supply, and announce I've averaged over 100mpg on the way there.
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Having made a space for two batteries in the rear, with two clamp-down arrangements, etc. - and having effectively ended up with a couple of 'spare' and 'free' batteries in the course of this process - I could just add a second rear battery when I go on a long trip, but for normal short journies leave it at home to save weight. (Another advantage of using DC/DC converters is that I can drop in ANY 12-ish volt battery to increase capacity, even if it is a different size, age, condition, etc. or even a completely different type, such as gel or LiFePO4, because the batteries can all be completely isolated from each other, and can be charged by different chargers.)
I have, for example, a cheap 90 Ah AGM battery that came with a 4-year warranty and which failed after a few months. It's the one I have now replaced with the Odyssey from another supplier. I don't want another of those cheap AGM's because they don't seem to last, and I can't get a cash refund, but I can return it and get them to send me a good gel battery for example which I can drop in on longer journies.
Quote:
Originally Posted by redpoint5
I'm no expert on DC->DC converters, but it seems like running 14v from the converter would have the effect of trying to charge your starter battery. A lead acid battery at near full charge is only 50% efficient at accepting a charge. The same goes for discharge.
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Yeah, kind of. 14.0v is more like a 'float' voltage than a properly efficient charging voltage. In fact, because when driving there is always a load of at least 6.4A, the actual voltage seen by the starter battery at the moment (may change after fitting a larger cable front to rear) tends to be no higher than about 13.5v, although if I leave the converter active while I'm parked up during the day (which I do sometimes if I know I've been depleting the starter battery more than usual) then when I come back the starter battery voltage will often show 14.0v.
I didn't know about fully-charged lead acid batteries being less efficient than partially charged ones. (Did I understand that right?). Are you saying they are also less efficient on discharge when full? The point of the DC/DC converter is more to prevent the starter battery from being discharged rather than to charge it up. In fact the starter battery is always slightly depleted as it is used for starting the engine, and also for any other peak current demands over and above 12A, such as use of electric window motors, wipers, blower, lights, indicators, etc.
I've deliberately set up the wiring for this system so that I can experiment. I've run (just been doing that today in fact and I'll complete tomorrow) ...I've just run a 35mm2 (2 guage AWG) cable through from rear to front so that I CAN connect rear and front batteries in a direct parallel arrangement, to see if that is more efficient. The current pair of batteries, although of slightly different types (both AGM but the Odyssey is pure lead) OUGHT to live happily together in direct parallel, sharing the same charger, etc. I might find I like running the system like that, in which case I can just use the DC/DC converter on longer trips with a third, drop-in, 'random freebie' battery - just whatever battery I have lying about. On longer trips I won't be carrying my tools in the back so there would be no issue of the rear end sinking low and looking temptingly full. (There's been a spate of thefts of tools from vehicles in my area recently so I worry about these things.)